1. Technical Field
This invention relates generally to conversion of a probability signal to a binary value, and is especially useful for encoding pictorial imagery for reproduction on binary display and/or printing systems.
2. Background Art
Binary displays and printers are capable of making a mark, usually in the form of a dot, of a given, uniform size and at a specified resolution in marks per unit length, typically dots per inch. It has been common to place the marks according to a variety of geometrical patterns such that a group of marks when seen by the eye gives a rendition of an intermediate color tone between the color of the background (usually white paper stock) and total coverage, or solid density. Continuous tone images are simulated by organizing groups of sub-elements into halftone cells which have gray level capabilities equal to the number of sub-elements in the cell plus one.
False contours are artifacts resulting from gray scale quantization steps which are sufficiently large to create a visible density step when the input image is truly a smooth, gradual variation from one to the other. Commonly assigned, co-pending U.S. patent application Ser. No. 459,496 filed in the names of Hwai T. Tai and Yee S. Ng et al. concurrently herewith is concerned with suppressing false contours in screened images. In that application, each sequential sub-element of a halftone cell is assigned a weighted probability of being "ON" in accordance with the gray level value of the associated pixel of the input signal. While the Ng patent application discloses one embodiment which incorporates a real time random number generator implementation to assign a weighted probability to each sub-element being "ON", real time random number generators are calculation intensive, and therefore time consuming.